South Dakota’s once-sleepy Senate race appears to be waking up – and it could throw a wrench into Republicans’ plans to seize control of the chamber next month.

The state’s Senate race for months had been considered a virtual lock for Republicans. But a new poll shows the independent in the race surging, and Democratic fundraisers have started to pour money into the contest.

Now, just as a late shake-up turned the Kansas Senate race competitive and prompted Republicans to rush to the incumbent’s aid, South Dakota could soon represent another Republican headache.

“I think the dynamics in South Dakota are putting pressure on Republicans,” said Nathan Gonzalez, managing editor of The Rothenberg Political Report.

The latest poll showed front-running former GOP Gov. Mike Rounds leading in South Dakota – but not by much.

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According to the Survey USA/KOTA/KSFY/Aberdeen American News poll taken between Oct. 1 and Oct. 6, Rounds is only leading independent Larry Pressler 35-32 percent among likely voters. (Pressler is a former GOP senator who has not said how he would caucus if elected.)

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Not far behind is Democrat Rick Weiland, with 28 percent.

Gonzalez said he’s not “completely surprised” over the poll findings, because “I didn’t think Rounds ever closed the deal in this race.”

But he added: “I do tend to be surprised when a third-party candidate is getting more than 20 percent.”

The survey results are raising eyebrows all over the political spectrum because the poll before it, commissioned by CBS/NYT/YouGov, had the former governor leading Pressler 42-12 percent in late September, with Weiland getting 27 percent. A Survey USA poll taken earlier that month showed Rounds with a comfortable double-digit lead ahead of both Pressler and Weiland, as did every other poll dating back to April.

So what’s happening?

Dick Wadhams, senior strategist for the Rounds campaign, told FoxNews.com the poll is an anomaly, and that “the race is not nearly as close as that.”

In fact, Rounds is still ahead with double digits, Wadhams insisted.

But the Democrats must sense the dynamics are the shifting, too, as they’re putting new resources into the race. FoxNews.com has confirmed the Democratic Senatorial Campaign Committee (DSCC) is devoting $1 million to the race – an investment that suggests they believe Pressler will peel just enough votes off Rounds to give their candidate a fighting chance. If either wins, that’s one fewer race Republicans can count on for the six total seats they need to seize control of the Senate.

“The DSCC must have their own data or else they wouldn’t be putting so much money into this,” Gonzalez said.

That money will add to a push of negative ads funded by other Democratic super PACs already weighing in on the race. According to reports, Every Voice Action started running TV ads on Sept. 18 criticizing Rounds. Meanwhile, Mayday, a super PAC co-founded by Harvard University law professor Lawrence Lessig, said it would start a $1 million campaign in supporting Weiland and against Rounds.

Rounds, for his part, has been dealing with questions over the so-called “EB-5 scandal,” which refers to a federal program that allowed immigrants to earn green cards by investing $500,000 in American businesses. Rounds expanded the program, which is now being scrutinized for corruption and abuse, during the tail-end of his 2003-2011 term.

“I think Rounds has not run a spectacular campaign and his fundraising has lagged,” added Gonzalez. “He’s let Pressler and Weiland define themselves, and I think he’s been averse to running negative or contrast ads,” allowing his “opponents to be whatever they want to be.”

In Pressler’s case, that’s a sage, moderate, three-term former senator who feels it’s his duty to get back to Washington for the sake of his state. Pressler, 72, has one paid staffer and only $100,000 in the bank, but his message must be resonating somewhere if the recent numbers are to be believed.

“I wouldn’t put too much stock in that,” Pressler told FoxNews.com, referring to the polls, “but I do feel we have never been so warmly received as we (he and his wife, Harriet) have been as we travel across the state.”

Pressler, who was first elected in 1978 and served three terms, was ousted by current Democratic Sen. Tim Johnson, who is retiring this year. A Vietnam War veteran, Pressler is known for being the only lawmaker approached in the infamous Abscam sting who deliberately turned down the offered bribe.

“It’s somewhat audacious, but I want to do public service,” said Pressler, now a grandfather of four. He wants to serve only one term in order to bring back revenues and jobs to South Dakota, and to help break the gridlock in Washington. He counts fellow independent Sen. Angus King, of Maine, as a friend, and has not yet decided if he would caucus with the Democrats or Republicans if he upsets Rounds on Nov. 4.

But Wadhams says that is not likely to happen and so far, the political tip sheets agree. While acknowledging that the dynamics seem to be shifting, Gonzalez says the Rothenberg Political Report still has the race “leaning Republican.” Cook Political Report, too, has the race in “likely” standing, between “leaning” and “solid” Republican.

“Larry Pressler has had a bit of a honeymoon in this campaign,” Wadhams said, suggesting the gloves in the Rounds corner are about to come off.

A similar shift has taken place in Kansas, where GOP Sen. Pat Roberts now faces a challenge from independent Greg Orman – and has hammered Orman as a liberal.

In South Dakota, Wadhams said: “Voters are going to find out that there are no fundamental differences between Weiland and Pressler, that a new, liberal version of Pressler has emerged. Anyone who says he has voted for Obama twice, who supports ObamaCare and gun control – there is something wrong with that.”

Pressler acknowledges voting for President Obama, and supports the Affordable Care Act, but with modifications. He also supports limited background checks on gun purchases, he said.

“I am a practical moderate,” he said. “I would say we have a very liberal Weiland and a very conservative Rounds. I am a moderate centrist and I want to work to solve problems.”